Assuming you have been in business for more than 12 months, you claim a WDA (Writing Down Allowance) of 25% p.a. - up to a maximum of Â£3000, as it considered an "expensive car", as it costs more than Â£12,000 (the actual depreciation would be given on disposal - either as a Balancing Allowance or Balancing Charge). This would be further restricted by 50% to reflect your private use; likewise the loan interest would be restricted by 50%.
As your turnover is below the VAT threshold, you can use the Inland Revenue's Approved Mileage Rates (40p per mile for the first 10,000 miles, and 25p thereafter) plus any finance interest instead of calculating the business % of all the relevant costs (including Capital Allowances).
Which is the better option will normally depend on the price of the car, its economy and reliability and the business miles travelled.
Have you read the Inland Revenue's Help Sheet IR222?
This gives guidance on calculating business profits, expenses, losses, accounting periods, Capital Alloances etc.
It is downloadable from their website.